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Observation of an Oxonium Ion Intermediate in Ethanol Dehydration to Ethene on Zeolite

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Observation of an Oxonium Ion Intermediate in Ethanol Dehydration to Ethene on Zeolite ARTICLE https://doi.org/10.1038/s41467-019-09956-7 OPEN Observation of an oxonium ion intermediate in ethanol dehydration to ethene on zeolite Xue Zhou1,2,4, Chao Wang1,4, Yueying Chu1,4, Jun Xu1,3, Qiang Wang 1, Guodong Qi1, Xingling Zhao1,2, Ningdong Feng1 & Feng Deng1 Zeolite-catalyzed dehydration of ethanol offers promising perspectives for the sustainable production of ethene. Complex parallel-consecutive pathways are proposed to be involved in 1234567890():,; the reaction network of ethanol dehydration on zeolites, where the initial step of ethanol dehydration is still unclear particularly for the favorable production of ethene at lower temperature. Here we report the observation of a triethyloxonium ion (TEO) in the dehy- dration of ethanol on zeolite H-ZSM-5 by using ex situ and in situ solid-state NMR spec- troscopy. TEO is identified as a stable surface species on the working catalyst, which shows high reactivity during reaction. Ethylation of the zeolite by TEO occurs at lower temperature, leading to the formation of surface ethoxy species and then ethene. The TEO-ethoxide pathway is found to be energetically preferable for the dehydration of ethanol to ethene in the initial stage, which is also supported by theoretical calculations. 1 National Centre for Magnetic Resonance in Wuhan, State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Key Laboratory of Magnetic Resonance in Biological Systems, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China. 2 University of Chinese Academy of Sciences, Beijing 100049, China. 3 Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China. 4These authors contributed equally: Xue Zhou, Chao Wang, Yueying Chu. Correspondence and requests for materials should be addressed to J.X. (email: [email protected]) or to F.D. (email: [email protected]) NATURE COMMUNICATIONS | (2019) 10:1961 | https://doi.org/10.1038/s41467-019-09956-7 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-09956-7 thene is one of the most important commodity chemicals, Diethyl ether (DEE) is concurrently produced with ethene, and Ewhich is currently produced by cracking processes from the kinetic and spectroscopic studies revealed that intermolecular petroleum. Due to the large availability of bioethanol from dehydration of ethanol leads to DEE via dimeric ethanol species renewable biomass sources and the dwindling of fossil resource1, or reaction of an ethoxy group with undissociated ethanol on the conversion of ethanol to ethene and higher hydrocarbons now zeolites13,22,23. The following cracking of DEE produces ethene, is receiving increasing attention from both academia and which occurs preferably at lower reaction temperatures as com- industry2,3. Among of the heterogeneous catalysts studied, zeo- pared to the unimolecular dehydration of ethanol to ethene14,24. lites especially ZSM-5 or modified ZSM-5 are the promising Note that the ethoxy species was generally proposed to be the catalysts as they can be tuned to exhibit higher activity than the direct precursor to ethene. However, the generation of such traditional alumina-based catalysts4–9: the reaction temperature is species is still not well understood. Thus, although different lower and a low concentration of ethanol aqueous solution can be reaction schemes have been proposed, the detailed mechanism of used. The formation of ethene is the first step in ethanol dehy- the initial step of ethanol dehydration remains elusive, particu- dration; subsequent polymerization, cracking and aromatization larly for the favorable production of ethene from DEE at lower by the secondary reaction of ethene leads to the formation of temperature. longer-chain hydrocarbons10, very similar to the formation of Here we report the observation and identification of reaction hydrocarbons in the conversion of methanol over zeolites11,12. intermediates in ethanol dehydration to ethene over zeolite The detailed knowledge of the reaction mechanism of ethene H-ZSM-5. The ex situ and in situ solid-state NMR spectroscopy formation in ethanol dehydration is important not only for allows for exploration of the dehydration of ethanol at the initial understanding ethanol dehydration as a model reaction of alcohol stage. A triethyloxonium ion (TEO) is observed as a potential conversion but also for the development of improved catalysts for active intermediate on the working catalyst. We provide evidence light olefins production. for the involvement of TEO in the formation of surface ethoxy The formation of ethene in the initial dehydration process has species and then ethene on the zeolite. The elementary reactions been experimentally13–15 and theoretically16–19 investigated. A leading to the formation of ethene are identified by combining parallel-consecutive route was proposed3: dehydration of ethanol experiments and theoretical calculations, which suggests that a to ethene and diethyl ether in parallel and a consecutive TEO-mediated reaction route is operative for the dehydration of decomposition of diethyl ether to ethene. In the unimolecular ethanol to ethene. dehydration of ethanol to ethene, it was proposed that the acidic proton transfers from the zeolite to the OH group on ethanol, which subsequently releases water to form an intermediate such Results as carbocation or an alkoxide bound to zeolite. Solid-state NMR Observation of TEO. H-ZSM-5 (Si/Al = 11.5, Zeolyst) was used and infrared spectroscopic studies16,20,21 confirmed the forma- for the dehydration of ethanol. The XRD and 27Al and 29Si solid- tion of surface-bound ethoxy species as a stable intermediate in state NMR spectra show structure information of the H-ZSM-5 ethanol dehydration of on H-ZSM-5 and HY zeolites. Ethene and catalyst (Supplementary Fig. 1 and 2). The acidic property was hydrocarbons are formed by the subsequent decomposition of the examined by FT-IR and NH3-TPD (Supplementary Fig. 3 and 4). surface ethoxy species. The reactions were conducted in a pulse-quench reactor25,in which ethanol was pulse-injected at temperatures ranging from 160 to 350 °C and allowed to react for 4 s under continuous He a carrier gas flow before the reaction was thermally quenched by 160°C pulsing liquid nitrogen onto the catalyst bed. The reaction fl X8 ef uent was analyzed by GC (Supplementary Fig. 5). Ethene is the * only product at 160–200 °C, indicating that the dehydration of ethanol to ethene is favorable at lower temperature. With 69.0 increasing reaction temperature, the secondary reaction of ethene b + 200°C 62.5 fi 85.0 leads to the formation of C3 products including ole ns and X8 * aromatics. Figure 1 shows the ex situ 13C CP/MAS NMR spectra of trapped products from the dehydration of 13C-1-enriched ethanol ° 13 13 – c 250 C (CH3 CH2OH, 99% C) on H-ZSM-5 at 160 350 °C for 4 s. X8 * Ethanol and DEE are observed as reflected by the strong signals at 62.5 and 69.0 ppm respectively, due to the 13C-enriched d 300°C methylene carbons; the methyl carbons produce the weak signals at 12–17 ppm. The adsorbed species formed at 200 °C were X8 * further analyzed by two-dimensional (2D) 1H–13C HETCOR NMR experiment (Supplementary Fig. 6). A weak signal at e 350°C 72.8 ppm that was seriously overlapped by the ethanol signal (69.0 ppm) in the 1D 13C NMR spectra was clearly discerned in 147.3 156.3 the 2D spectrum, which can be ascribed to the methylene carbon 243.6 255.7 249.6 32.2 29.2 47.6 126.5 9.7 X8 17.0 of ethoxy group. The formation of the alkoxy species was well * established in methanol and ethanol dehydration on acidic zeolites. Hunger et al.20 reported the surface ethoxy species from 250 200 150 100 50 0 ppm ethanol dehydration on HY zeolite, which has a chemical shift of 72.6 ppm. Fig. 1 13C NMR analysis of the formed surface species on zeolite. 13C CP/ Interestingly, a well-resolved signal appears at 85.0 ppm (Fig. 1) MAS NMR spectra of trapped species on H-ZSM-5 obtained from reaction and grows up at intermediate temperature. The oligomeric alkoxy 13 of CH3 CH2OH for 4 s at different temperatureof 160 °C–350 °C (a-e). species formed by the oligomerization of ethene on acidic zeolites Asterisks denote spinning sidebands were previously proposed to produce broad signals at 78.0 to 2 NATURE COMMUNICATIONS | (2019) 10:1961 | https://doi.org/10.1038/s41467-019-09956-7 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-09956-7 ARTICLE 20,26 90.0 ppm in the literatures . Since there is no any C3+ species TEO and the ethoxy species are solely formed and located on formed at this reaction condition, the oligomerization of ethene Brønsted acid sites for charge compensation, while DEE and did not likely occur in our case. Furthermore, no methylene ethanol are trapped over zeolite via either chemical or physical signals at 20.0 to 40.0 ppm can be observed that are supposed to adsorption. Considering the total Brønsted acid sites (about be oligomerized compounds27. Therefore, the 85.0 ppm signal 1.03 mmol/g) on the catalyst, the amount (less than 0.23 mmol/g) should not be ascribed to the oligomeric alkoxy species. Instead, of DEE adsorbed on Brønsted acid sites should be lower than that we proposed on the basis of its chemical shift the formation of (0.34 mmol/g) of TEO at this reaction condition. At temperature alkyl-substituted oxonium ions. Munson and Haw reported the above 300 °C (Fig. 2d, e), the 85.0 ppm signal is almost formation of trimethyloxonium ion (TMO) with a characteristic unobservable, which can be ascribed to the decomposition of 13C NMR signal at 80.0 ppm by the reaction of dimethyl ether on TEO.
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